The present invention relates to a process for recycling the filter dust obtained during the production of steel in an electric arc furnace.
A known process for producing steel is the process which is carried out in an electric arc furnace.. In an electric arc furnace, the steel is principally molten from scrap iron or sponge iron.
During the implementation of this steel production process, considerable quantities of dust are produced. This dust is collected in dust-filtering devices. Depending on the conditions of operation of the process and on the raw materials used, between 3 and 30 kg of filter dust are produced per ton of steel. The filter dust obtained is essentially composed of metal oxides: iron, zinc, calcium, aluminium, lead, bismuth, mercury, cadmium, tin and silicium. Given that the filter dust contains valuable metals, in particular zinc, lead and cadmium in oxide form, efforts are made to treat the filter dust, for example by a mixing process using a rotary furnace, in order to particularly enrich the zinc, lead and cadmium metals. However, a large quantity of energy is needed, which is economically justified only if, for example, the proportion of zinc oxides in the filter dust exceeds 25%. If such proportions are not contained in the filter dust, said dust should, in view of the toxic heavy metals that it contains, be put into classified dump ground, which entails considerable extra costs.
It is known from the article published in "Fachberite Huttenpraxis Metallweiterverarbeitung" Vol. 25, No. 1, 1987, pages 16, 18, to inject small coal into the molten metal bath of an electric arc furnace, using a lance sunk into said molten metal bath in order to obtain a foaming of the slag.
The revue "Stahl und Eisen" 109 (1989), No. 7, pages 359-365 describes the pneumatic recycling of filter dust in electric arc furnaces by injection of said dust in the molten metal bath. The object of this is to enrich the newly produced dust in zinc and in lead. However, the obtained samples of steel partially show high zinc contents, which are undesirable. Moreover, it is pointed out that the injection of filter dust requires more energy for melting and reduction of said dust during the production of steel in the electric arc furnace.
Patent Application DE-A-3 708 730 describes a process for re-using the filter dust obtained during the production of steel in a converter, which is not an electric arc furnace. According to this process, the filter dust is injected in the melting tank with carbon carriers, for example small coal, vehiculated by a carrier gas, such as carbon monoxide or methane. It is expressly indicated that it is important to inject the filter dust in the molten metal bath, i.e. through nozzles situated below the surface of the bath.